363 THE EVOLUTION OF KAN. 



EXPLANATION OF PLATES VI. AND VII. 



Plates VI. and VII. are meant to represent the more or less complete 

 agreement, as regards the most important relations of form, between the 

 embryo of Man and that of other Vertebrates in early stages of individual 

 development. This agreement is the more complete, the earlier the period 

 at which the human embryo is compared with those of other Vertebrates. 

 It is retained longer, the more nearly related in descent the respective 

 matured animals are — corresponding to the " law of the ontogenetic con- 

 nection of systematically related forms." (Cf. Chapter XII., p. 366.) 



Plate VI. represents the embryos of two of the lower, and two of the 

 higher Vertebrates in three different stages : of a Fish (Osseous-fish, F); of 

 an Amphibian (Land-salamander, j8) ; of a Reptile (Tortoise, T) ; and of a 

 Bird (Chick, C). 



Plate VIII. shows the embryos of four Mammals in the three correspond- 

 ing stages : of a Hog (H), Calf (C), Rabbit (R), and a Man (M). The con- 

 ditions of the three different stages of development, which the three cross- 

 vows (I., II., III.) represent, are selected to correspond as exactly as possible. 



The first, or upper cross-row, I., represents a very early stage, with gill- 

 openings, and without limbs. The second (middle) cross-row, II., shows a 

 somewhat later stage, with the first rudiments of limbs, while the gill- 

 openings are yet retained. The third (lowest) cross-row, III., shows a still 

 later stage, with the limbs more developed and the gill-openings lost. The 

 membranes and appendages of the embryonic body (the amnion, yelk-sac, 

 allantois) are omitted. The whole twenty-four figures are slightly magnified, 

 the upper ones more than the lower. To facilitate the comparison, they are all 

 reduced to nearly the same size in the cuts. All the embryos are seen from 

 the left side ; the head extremity is above, the tail extremity below ; the 

 arched back turned to the right. The letters indicate the same parts, in 

 all the twenty-four figures, namely : v, fore-brain ; z, twixt-brain ; m, mid- 

 brain ; h, hind-brain; n, after-brain; r, spinal marrow; e, nose; o, eye; 

 0, ear ; fc, gill-arches j y, heart j w, vertebral column ; /, fore-limbs ; fc, bind, 

 limbs 1 «, taiL 1 ** 



